Rotary shredders are used for shredding a variety of materials such as paper, cardboard, plastic film, cloth, webbing, textile fibers of natural or synthetic material, and others. European Patent EP 419 919 B1 describes a shredder for such materials that includes a rotor having a plurality of circumferential ribs spaced apart along its length and a counter knife having teeth axially aligned with the valleys or grooves defined between the ribs of the rotor. A plurality of cutters are mounted in pockets formed in the outer surface of the rotor. Each cutter has two faces that are at a right angle to each other and form a V-shape that meshes with a correspondingly V-shaped recess between two adjacent teeth of the counter knife. Material fed into the space between the rotor and counter knife is cut into pieces by the cutters and the pieces pass through a screen that surrounds a portion of the circumference of the rotor; pieces too large to pass through the screen are carried by the rotor back to the counter knife to be cut again.
When rotary shredders such as that described in EP 419 919 are used for shredding certain “stringy” types of high tensile-strength material (such as fibers of pre-stressed polypropylene), it has been found that the material tends to wrap around the rotor and accumulate in the valleys of the rotor surface. Once the material is laying in the valleys, it cannot be cut between the rotor cutters and the counter knife. The material wrapped around the rotor increases the friction between the rotor and the counter knife as well as between the rotor and the screen, leading to high stresses on the machine, high energy consumption, and frequent overload and jamming of the machine.